"We are afraid that if the country's current leadership is removed from power unconstitutionally, then the opposition and today's leadership may simply change places," the Interfax news agency quoted Putin as saying.

"One will become (the new) leadership and the other -- the opposition."

Putin warned that in that case "a civil war will stretch on for who knows how long," Interfax reported.

The Russian leader's comments came as fighting raged in the main Syrian cities of Damascus and Aleppo on Monday.